The present invention relates generally to a combustion chamber, particularly to a gas turbine engine combustion chamber.
In order to meet the emission level requirements, for industrial low emission gas turbine engines, staged combustion is required in order to minimise the quantity of the oxide of nitrogen (NOx) produced. Currently the emission level requirement is for less than 25 volumetric parts per million of NOx for an industrial gas turbine exhaust. The fundamental way to reduce emissions of nitrogen oxides is to reduce the combustion reaction temperature, and this requires premixing of the fuel and a large proportion, preferably all, of the combustion air before combustion occurs. The oxides of nitrogen (NOx) are commonly reduced by a method, which uses two stages of fuel injection. Our UK patent no. GB1489339 discloses two stages of fuel injection. Our International patent application no. WO92/07221 discloses two and three stages of fuel injection. In staged combustion, all the stages of combustion seek to provide lean combustion and hence the low combustion temperatures required to minimise NOx. The term lean combustion means combustion of fuel in air where the fuel to air ratio is low, i.e. less than the stoichiometric ratio. In order to achieve the required low emissions of NOx and CO it is essential to mix the fuel and air uniformly.
The industrial gas turbine engine disclosed in our International patent application no. WO92/07221 uses a plurality of tubular combustion chambers, whose axes are arranged in generally radial directions. The inlets of the tubular combustion chambers are at their radially outer ends, and transition ducts connect the outlets of the tubular combustion chambers with a row of nozzle guide vanes to discharge the hot gases axially into the turbine sections of the gas turbine engine. Each of the tubular combustion chambers has two coaxial radial flow swirlers, which supply a mixture of fuel and air into a primary combustion zone. An annular secondary fuel and air mixing duct surrounds the primary combustion zone and supplies a mixture of fuel and air into a secondary combustion zone.
One problem associated with gas turbine engines is caused by pressure fluctuations in the air, or gas, flow through the gas turbine engine. Pressure fluctuations in the air, or gas, flow through the gas turbine engine may lead to severe damage, or failure, of components if the frequency of the pressure fluctuations coincides with the natural frequency of a vibration mode of one or more of the components. These pressure fluctuations may be amplified by the combustion process and under adverse conditions a resonant frequency may achieve sufficient amplitude to cause severe damage to the combustion chamber and the gas turbine engine. Alternatively the amplitude of the pressure fluctuations may be sufficiently large such as to induce damage to the combustion chamber and the gas turbine engine in their own right.
It has been found that gas turbine engines, which have lean combustion, are particularly susceptible to this problem. Furthermore it has been found that as gas turbine engines which have lean combustion reduce emissions to lower levels by achieving more uniform mixing of the fuel and the air, the amplitude of the resonant frequency becomes greater. It is believed that the amplification of the pressure fluctuations in the combustion chamber occurs because the heat released by the burning of the fuel occurs at a position in the combustion chamber, which corresponds, to an antinode, or pressure peak, in the pressure fluctuations.
Our European patent application No. 00311040.0 filed Dec. 11, 2000, which claims priority from UK patent application 9929601.4 filed Dec. 16, 1999 discloses a combustion chamber arranged to reduce this problem. The combustion chamber has at least one fuel and air mixing duct for supplying a fuel and air mixture to a combustion zone in the combustion chamber. Fuel injection means is arranged to supply fuel into the at least one fuel and air mixing duct. Air injection means is arranged to supply air into the at least one fuel and air mixing duct. The air injection means comprises a plurality of air injectors spaced apart in the direction of flow through the at least one fuel and air mixing duct to reduce the magnitude of the fluctuations in the fuel to air ratio of the fuel and air mixture supplied into the at least one combustion zone.
However, although the fuel to air ratio fluctuations have been reduced there is a risk of auto ignition of the fuel in the fuel and air mixing duct in the wakes from the air injectors due to the possibility of excessively long residence times in the fuel and air mixing duct. The risk of excessively long residence time is a function of the gas turbine engine pressure ratio. The higher the pressure ratio, the higher the risk of autoignition.
Accordingly the present invention seeks to provide a combustion chamber which reduces or minimises the above-mentioned problem.
Accordingly the present invention provides a combustion chamber comprising at least one combustion zone defined by at least one peripheral wall, at least one fuel and air mixing duct for supplying a fuel and air mixture to the at least one combustion zone, the at least one fuel and air mixing duct having an upstream end and a downstream end, fuel injection means for supplying fuel into the at least one fuel and air mixing duct, air injection means for supplying air into the at least one fuel and air mixing duct, the pressure of the air supplied to the at least one fuel and air mixing duct fluctuating, the air injection means comprising a plurality of air injectors spaced apart transversely to the direction of flow through the at least one fuel and air mixing duct, each air injector comprising a slot extending in the direction of flow through the at least one fuel and air mixing duct to reduce the magnitude of the fluctuations in the fuel to air ratio of the fuel and air mixture supplied into the at least one combustion zone.
Preferably the at least one fuel and air mixing duct comprises at least one wall, the air injectors comprise a plurality of slots extending through the wall.
Preferably the combustion chamber comprises a primary combustion zone and a secondary combustion zone downstream of the primary combustion zone.
Preferably the combustion chamber comprises a primary combustion zone, a secondary combustion zone downstream of the primary combustion zone and a tertiary combustion zone downstream of the secondary combustion zone.
The at least one fuel and air mixing duct may supply fuel and air into the primary combustion zone. The at least one fuel and air mixing duct may supply fuel and air into the secondary combustion zone. The at least one fuel and air mixing duct may supply fuel and air into the tertiary combustion zone.
The at least one fuel and air mixing duct may comprise a single annular fuel and air mixing duct, the air injection means being circumferentially spaced apart and the air injection means extending axially. The annular fuel and air mixing duct may comprise an inner annular wall and an outer annular wall, the fuel injector means being provided in at least one of the inner and outer annular walls. The air injector means may be arranged in the inner and outer annular walls. The air injection means in the inner annular wall may be staggered circumferentially with respect to the air injection means in the outer annular wall.
Preferably the fuel and air mixing duct comprises a radial fuel and air mixing duct, the air injection means being circumferentially spaced apart and the air injection means extending radially. Preferably the radial fuel and air mixing duct comprises a first radial wall and a second radial wall, the air injector means being provided in at least one of the first and second radial walls. Preferably the air injector means are provided in the first and second radial walls. The air injection means in the first radial annular wall may be staggered circumferentially with respect to the air injection means in the second radial wall.
Alternatively the fuel and air mixing duct comprises a tubular fuel and air mixing duct, the air injector means being circumferentially spaced apart.
Preferably the fuel injector means is arranged at the upstream end of the fuel and air mixing duct and the air injector means are arranged downstream of the fuel injector means.
Alternatively the fuel injector means is arranged between the upstream end and the downstream end of the at least one fuel and air mixing duct, a portion of the air injector means are arranged upstream of the fuel injector means and a portion of the air injector means are arranged downstream of the fuel injector means.
Preferably each air injector means at the downstream end of the fuel and air mixing duct is arranged to supply more air into the fuel and air mixing duct than said air injector means at the upstream end of the fuel and air mixing duct.
Preferably each air injector means at a first position in the direction of flow through the fuel and air mixing duct is arranged to supply more air into the fuel and air mixing duct than said air injector means upstream of the first position in the fuel and air mixing duct.
Preferably each air injector means at the first position in the fuel and air mixing duct is arranged to supply less air into the fuel and air mixing duct than said air injector means downstream of the first position in the fuel and air mixing duct.
Preferably the volume of the fuel and air mixing duct being arranged such that the average travel time from the fuel injection means to the downstream end of the fuel and air mixing duct is greater than the time period of the fluctuation.
Preferably the volume of the fuel and air mixing duct being arranged such that the length of the fuel and air mixing duct multiplied by the frequency of the fluctuations divided by the velocity of the fuel and air leaving the downstream end of the fuel and air mixing duct is at least one.
Preferably the volume of the fuel and air mixing duct being arranged such that the length of the fuel and air mixing duct multiplied by the frequency of the fluctuations divided by the velocity of the fuel and air leaving the downstream end of the fuel and air mixing duct is at least two.
Preferably the plurality of air injectors extend in the direction of flow through the at least one fuel and air mixing duct over a length equal to half the wavelength of the fluctuations of the air supplied to the at least one fuel and air mixing duct.
Preferably the length of an air injector in the direction of flow through the at least one fuel and air mixing duct multiplied by the frequency of the fluctuations divided by the velocity of the fuel and air inside the at least one mixing duct is at least one.
Preferably the length of an air injector in the direction of flow through the at least one fuel and air mixing duct multiplied by the frequency of the fluctuations divided by the average velocity of the fuel and air inside the at least one mixing duct is at least two.
Preferably the at least one fuel and air mixing duct comprises a swirler. Preferably the swirler is a radial flow swirler.
The present invention also provides a fuel and air mixing duct for a combustion chamber, the fuel and air mixing duct comprising fuel injection means for supplying fuel into the fuel and air mixing duct, air injection means for supplying air into the fuel and air mixing duct, the air injection means comprising a plurality of air injectors spaced apart transversely to the direction of flow through the fuel and air mixing duct, the air injectors comprise a plurality of slots extending in the direction of flow through the fuel and air mixing duct.